Precious transition metal ions and their coordination complexes find industrial applications as supported catalysts and performance chemicals in fine chemicals industries. For example metals like Ag, Au, Pd, Pt and Rh are used in a variety of industrial applications as catalysts for oxidation, hydrogenation and dehydrogenation reactions. Palladium salt catalysts are also widely used in catalyzing such reactions as oxidation, dehydrogenation, hydrogenation, isomerization and dimerization. The efficient recovery and purification of platinum group metals, such as palladium and platinum, from spent catalyst is economically desired. Consequently, lots of research efforts are directed to develop processes for complete recovery of such metals from spent catalysts worldwide. Various polymeric materials, modified silica, zeolite and/or clay materials are used as support for these metals. Similarly, coordination metal complexes of Pd, Pt, Ru and Rh are used as commercial catalysts in homogeneous conditions for hydroformylation and hydrogenation reactions. However, owing to difficulties in separation of metal complexes from product mixtures, there is growing need to developed heterogenized catalysts where coordination metal complexes are supported on polymeric or inorganic solid support like silica, carbon, zeolite, and alumina. Commercially, it is important to recover the precious metals from the support to the maximum extent possible once the catalyst is deactivated.
In certain situations, the catalysts are deactivated during the reaction cycle or subsequent work-up and the reaction effluent may comprise of various remnants, e.g., complexes of various valence states for the metal ions. In many cases, the presence of excessive contaminants reduces the efficacy of recycling of the catalyst. However, the problem related to degradation of catalysts over the repeated cycles, contaminants arise from side reaction and leaching of the effective catalyst for the solid support are common in some cases of heterogeneous catalysts. In view of the environmentally stipulated restrictions on disposal of metal containing waste, effective recovery of the residual precious metals from the reaction effluent is of paramount importance for a process to be environmentally acceptable and economically viable.
Furthermore, during the recovery of precious metals from ore or scrap including spent catalysts, the use of solvent extraction to separate the precious metals from one another and from base metals that may also be present is becoming more widespread. The hydrometallurgical processes employed for the separation and recovery of the platinum group metals, (e.g., platinum, palladium and rhodium), typically involve dissolving the metal ions by some type of oxidative acidic chloride leach, typically with aqua regia or hydrochloric acid/chlorine gas followed by solvent extraction.
Reference is made to the U.S. Pat. No. 4,705,896 (1987) to Van Der Puy et al., who have disclosed the extractants for the recovery of palladium using ortho alkoxy substituted phenyl oxime compounds which are useful for selectively separating and recovering palladium from aqueous compositions and mixtures containing palladium and other metals. In this patent, the oximes employed for the extraction of palladium are hydroxyoxime and derivatives, which are expensive considering the industrial application.
U.S. Pat. No. 4,654,145 (1987) to Demopoulos, et al., discloses the recovery of precious metals, particularly gold and the platinum group metals, from metal chloride solutions, by solvent extraction and selective stripping/precipitation. The active extractant used was, in particular alkyl substituted 8-hydroxyquinoline. Gold is precipitated either by hot water, or by hydrogen after a cold water wash. Palladium in the organic phase is precipitated either by hydrogen reduction or is stripped with mineral acid. Platinum stripped into aqueous solution can be recovered by hydrogen reduction in stages. The organic phase is not degraded providing acid is washed out before any hydrogen reduction, and can be recycled. The process required washing out of acid from the organic phase before solvent can be recover and reuse which is a multi step process.
G.B. Patents No. A-2127001, 1984 to Hunter William, discloses a process for recovering, precious metals viz. gold, palladium and platinum from a solution of at least one dissolved compound of that precious metal. The process comprises the adsorption of one precious metal on the activated carbon, wherein said activated carbon is in the form fibers. The loaded fibrous body is treated with aqueous cyanide solution to recover the metal ion. The main limitation of the process is that cyanide ligand must have high affinity for precious metals to be recover.
U.S. Pat. No. 4,578,250 (1986) to Dimmit, et al., discloses the separation and purification of palladium present in a aqueous solution with at least one other platinum group metal by adjusting the pH of the solution from 0 to 5, followed by treating the acidified solution with an ortho alkoxy substituted phenyl oxime by using solvent extractions techniques. Illustrative of useful solvents are aromatic, aliphatic and cyclo-aliphatic hydrocarbons such as toluene, cyclohexane, xylenes, chlorinated hydrocarbons and kerosene. The preferred solvent is kerosene. The aqueous phase was separated from the organic phase, followed by stripping of palladium from the association with the oxime compounds in the organic phase by extraction with an aqueous ammonia solution. The drawback of the process is that it involves multistage processes. Moreover recovery of solvent for further reuses makes the process complicated.
U.S. Pat. No. 4,435,258 (1984) to Melka, Jr., et al., discloses the recovery of palladium from spent electroless catalytic baths by dissolving the palladium to form a true solution followed by electro-deposition employing a nickel anode and a nickel or copper cathode. The recovery includes the steps of (a) dissolving the colloidal palladium in the bath so as to form a true solution by the addition of an oxidizing agent, e.g., hydrogen peroxide, (b) heating the bath to a temperature and for a time sufficient to essentially remove excess hydrogen peroxide, (c) placing the solution in an electrolytic cell having (i) a nickel anode, and (ii) a cathode comprised of a metal or metallic surface which is non-contaminating or easily separable from the palladium to be deposited; and (d) electrowinning palladium from the solution onto the cathode at a voltage which tends to minimize and substantially reduce tin deposits.
It has been found that with continued use, the catalytic bath becomes contaminated with copper from the copper cladding. When contamination reaches an extent such that the bath becomes ineffective or the electroless plating becomes less adherent than desirable, and must then be discarded, as waste hence may not be suitable for commercial application.
U.S. Pat. No. 4,105,742 (1978) to Edwards, et al., for the process for the separation of platinum and/or palladium from acid starting solutions containing the chlorocomplexes thereof together with other platinum group metals and/or base metals in 0.01 to 2M acid solutions comprises contacting the starting solution with a suitable extractant; the extractant soluble in water-immiscible solvent, carried in an organic phase and comprising functional groups of the formula R2N—CH2—COOH wherein R is a long-chain alkyl group and thereafter separating the two phases and recovering the extracted platinum or palladium or both from the loaded solvent extractant. These extractant are generally very expensive and use organic solvents that are not environment friendly, are not suitable for industrial application.
C. S. Patent No. 251467 (1988); to Pauko Jan, (Chemical Abstract 109 (24): 213964) disclose the recovery of palladium from wastewater by sorption on activated carbon, pre-treated with an alkali metal salt of Ethylenediaminetetraacetic acid (EDTA). The wastewater is required to be acidified to pH 3-7.5 before treating with activated carbon with EDTA solution to recover precious metals. However, this is useful for homogenous aqueous catalyst waste and is not directly applicable to non-aqueous systems.
JP Patent No. 54 9597 (1978) discloses the regeneration and recovery of palladium from spent residue, obtained from a catalytic process performed in homogeneous conditions, which is subjected to heating ca. 550 to 600° C. to decompose the organic fragment. The residue is then acid treated to obtain corresponding palladium salt. However, this invention has a limited scope in the sense that it is only concerned with degradation of metal complex having lower aliphatic mono-carboxylic acids as the organic moiety.
U.S. Pat. No. 4,331,634 (1982) to Sbanton; Kenneth J. and Grant; Richard A., wherein strongly acidic solution of sulphuric, hydrochloric, perchloric or nitric acid is used as stripping solvent for the extraction of the palladium from the organic acidic solution containing oxime as extracting reagent. The organic phase may also contain an anionic phase transfer material or catalyst to aid the extraction process. In this patent, the oximes employed for the extraction of palladium are hydroxyoxime and derivatives, which are expensive considering the industrial application.
Y. Baba, K. Inoue, K. Yoshizuka and T Furusawa, (Industrial Engineering Chemistry-Research, Volume 29, (1990), page 2111) have described the use of non-cheating oximes such as dodecanal oxime, decanal oxime, octanal oxime and hexanol oxime for the extraction of palladium metal. This report also has drawback in the sense that the oximes employed for the extraction are expensive considering the industrial application
According to above prior art, most of the metal recovery methods are applicable to the homogeneous reaction mixtures and employ chelating agents for extracting metal ions. Process known in the prior art for recovery of metals from supported catalysts also makes use of mineral acids and invariably destroyed the support structure.